An excellent aphorism showing subtle distinction between the two (1,4,4)

'an excellent aphorism' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'subtle distinction between the two' is the wordplay.
I cannot really understand how this works, but
'subtle' could be 'fine' (synonyms) and 'fine' is found in the answer.
'distinction' could be 'line' (line is a kind of distinction) and 'line' is found within the answer.
A single letter 'a' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

'showing' is the link.

Can you help me to learn more?